Fix bug that lead to wrong payment amount when switching payment method to PayPal later

src/pretix/base/payment.py

@@ -1,7 +1,7 @@
 from collections import OrderedDict
 from datetime import date
 from decimal import Decimal
-from typing import Any, Dict
+from typing import Any, Dict, Union
 
 import pytz
 from django import forms
@@ -266,7 +266,7 @@ class BasePaymentProvider:
         """
         raise NotImplementedError()  # NOQA
 
-    def checkout_prepare(self, request: HttpRequest, cart: Dict[str, Any]) -> "bool|str":
+    def checkout_prepare(self, request: HttpRequest, cart: Dict[str, Any]) -> Union[bool, str]:
         """
         Will be called after the user selects this provider as his payment method.
         If you provided a form to the user to enter payment data, this method should
@@ -394,14 +394,14 @@ class BasePaymentProvider:
         """
         return False
 
-    def retry_prepare(self, request: HttpRequest, order: Order) -> "bool|str":
+    def retry_prepare(self, request: HttpRequest, order: Order) -> Union[bool, str]:
         """
         Deprecated, use order_prepare instead
         """
         raise DeprecationWarning('retry_prepare is deprecated, use order_prepare instead')
         return self.order_prepare(request, order)
 
-    def order_prepare(self, request: HttpRequest, order: Order) -> "bool|str":
+    def order_prepare(self, request: HttpRequest, order: Order) -> Union[bool, str]:
         """
         Will be called if the user retries to pay an unpaid order (after the user filled in
         e.g. the form returned by :py:meth:`payment_form`) or if the user changes the payment
@@ -409,6 +409,10 @@ class BasePaymentProvider:
 
         It should return and report errors the same way as :py:meth:`checkout_prepare`, but
         receives an ``Order`` object instead of a cart object.
+
+        Note: The ``Order`` object given to this method might be different from the version
+        stored in the database as it's total will already contain the payment fee for the
+        new payment method.
         """
         form = self.payment_form(request)
         if form.is_valid():
@@ -458,7 +462,7 @@ class BasePaymentProvider:
         return '<div class="alert alert-warning">%s</div>' % _('The money can not be automatically refunded, '
                                                                'please transfer the money back manually.')
 
-    def order_control_refund_perform(self, request: HttpRequest, order: Order) -> "bool|str":
+    def order_control_refund_perform(self, request: HttpRequest, order: Order) -> Union[bool, str]:
         """
         Will be called if the event administrator confirms the refund.
 
@@ -523,7 +527,7 @@ class FreeOrderProvider(BasePaymentProvider):
     def order_control_refund_render(self, order: Order) -> str:
         return ''
 
-    def order_control_refund_perform(self, request: HttpRequest, order: Order) -> "bool|str":
+    def order_control_refund_perform(self, request: HttpRequest, order: Order) -> Union[bool, str]:
         """
         Will be called if the event administrator confirms the refund.
 

src/pretix/plugins/paypal/views.py

@@ -57,7 +57,19 @@ def success(request, *args, **kwargs):
 @event_view(require_live=False)
 def abort(request, *args, **kwargs):
     messages.error(request, _('It looks like you canceled the PayPal payment'))
-    return redirect(eventreverse(request.event, 'presale:event.checkout', kwargs={'step': 'payment'}))
+
+    if request.session.get('payment_paypal_order'):
+        order = Order.objects.get(pk=request.session.get('payment_paypal_order'))
+    else:
+        order = None
+
+    if order:
+        return redirect(eventreverse(request.event, 'presale:event.order', kwargs={
+            'order': order.code,
+            'secret': order.secret
+        }) + ('?paid=yes' if order.status == Order.STATUS_PAID else ''))
+    else:
+        return redirect(eventreverse(request.event, 'presale:event.checkout', kwargs={'step': 'payment'}))
 
 
 @csrf_exempt

src/pretix/presale/views/order.py

@@ -320,20 +320,21 @@ class OrderPayChangeMethod(EventViewMixin, OrderDetailMixin, TemplateView):
                 request.session['payment'] = p['provider'].identifier
                 request.session['payment_change_{}'.format(self.order.pk)] = '1'
 
+                new_fee = p['provider'].calculate_fee(self._total_order_value)
+                self.order.payment_provider = p['provider'].identifier
+                self.order.payment_fee = new_fee
+                self.order.total = self._total_order_value + new_fee
+                self.order._calculate_tax()
+
                 resp = p['provider'].order_prepare(request, self.order)
                 if resp:
                     with transaction.atomic():
-                        new_fee = p['provider'].calculate_fee(self._total_order_value)
                         self.order.log_action('pretix.event.order.payment.changed', {
                             'old_fee': self.order.payment_fee,
                             'new_fee': new_fee,
                             'old_provider': self.order.payment_provider,
                             'new_provider': p['provider'].identifier
                         })
-                        self.order.payment_provider = p['provider'].identifier
-                        self.order.payment_fee = new_fee
-                        self.order.total = self._total_order_value + new_fee
-                        self.order._calculate_tax()
                         self.order.save()
 
                         i = self.order.invoices.filter(is_cancellation=False).last()